How the hand has shaped sign languages

In natural languages, biological constraints push toward cross-linguistic homogeneity while linguistic, cultural, and historical processes promote language diversification. Here, we investigated the effects of these opposing forces on the fingers and thumb configurations (handshapes) used in natural sign languages. We analyzed over 38,000 handshapes from 33 languages. In all languages, the handshape exhibited the same form of adaptation to biological constraints found in tasks for which the hand has naturally evolved (e.g., grasping). These results were not replicated in fingerspelling—another task where the handshape is used—thus revealing a signing-specific adaptation. We also showed that the handshape varies cross-linguistically under the effects of linguistic, cultural, and historical processes. Their effects could thus emerge even without departing from the demands of biological constraints. Handshape’s cross-linguistic variability consists in changes in the frequencies with which the most faithful handshapes to biological constraints appear in individual sign languages.

Supplementary Information A2. Sources consulted for signs Websites of the online dictionaries listing the signs in each of the 33 sign languages that we analyzed. Lexical searched were conducted using written English translations, with the exception of a few sign languages (Danish, Norwegian, Mexican) for which native speakers were consulted for translations in other written languages than English.  (index and ring fingers, index and little fingers, middle and little fingers) The next two tables report the percentages, in 33 languages, of handshapes in which the fingers in the pair were selected (i.e., shaped differently compared to the other two fingers and the thumb that were closed; Table A), or identically shaped (Table B).

Supplementary Information B
The pictures included in Supplementary Material B illustrate the 160 distinct handshapes identified in the 28,343 signs examined in 33 languages. Each picture shows a screenshot taken from a video available on one of the websites listed in Supplementary Material A2. The screenshots were edited to enhance the presentation of the handshape. While the handshapes were clearly visible on the videos, details may not be discernable in the picturesmissing information is supplied by the descriptions accompanying the pictures. In pictures in which the two hands make different handshapes, we intend to illustrate the handshape of the right hand. We used the terminology of the Hamburg Notation System, after which our analysis was modelled, with the exception of thumb position that was classified using the terms from biomechanics and illustrated in Figure 1. A further terminological note: digits refers to the thumb and the four fingers. The presentation of the handshapes is organized according to the number of selected digits and the shape of the fingers and the thumb, features that were analyzed in the study. The handshapes are grouped in categories within which they share common features. Complex handshapes requiring detailed descriptions are presented separately. Percentages refer to the occurrences of the handshapes in the corpus of 28,343 signs.

All fingers selected
Fingers are identically shaped. A primary distinction among the handshapes in which all fingers are selected and identically shaped concerns whether or not the fingers make contact with the thumb.

1.1a Thumb-finger contact
The thumb makes contact with multiple fingers or (primarily) with the index finger. Finger shape varies -fingers are all closed, bent, flattened, or stacked.

1.1b Complex handshapes
In A, the thumb is in between the fingers. In B, the fingers are stacked and the thumb is extended.

No thumb-finger contact
Fingers are all selected and identically shaped and do not make contact with the thumb. They vary depending on whether or not they are spread. Their shape also varies -they are extended, bent, flattened, or hooked. The thumb is extended, abducted, adducted, or flexed.

1.2.a Spread fingers
Note: The picture of the handshape with flattened fingers and flexed thumb is missing.

One selected finger or selected thumb
The selected finger or the selected thumb is shaped differently compared to the other digits. Shapes of the selected and non-selected digits vary.

Closed non-selected digits
One finger or the thumb is shaped differently compared to the other digits that are closed. The thumb is either extended or bent. Each of the fingers can be selected in this type of handshapes. All selected fingers can be extended; only the index and the little fingers are shaped in additional configurations when selected.

Non-selected digits are not closed
Only the index finger or the middle finger is selected in this type of handshapes. The index finger and the middle finger are bent or flattened. The non-selected fingers are all extended, but vary as to whether or not they are spread. The thumb is either extended or adducted.

2.2a Spread non-selected fingers
2.2b Non-selected fingers are not spread In both of these handshapes, the selected finger -either the index finger (A) or the middle finger (B) -is bent. The thumb is adducted in A, extended in B.

Complex handshapes
The selected finger in these handshapes is either the thumb, the index finger, or the little finger.
-Selected thumb. The thumb is bent, contrasting to the fingers that are extended.
-Selected index finger. The shape of the index finger varies in these handshapes -it is extended in A and B, bent in C. Non-selected fingers vary in shape as well -they are flattened in A and B, bent in C. The thumb is adducted in A. The fingertip of the thumb is opposed to the fingertips of the non-selected fingers in B and C. -Selected little finger. The little finger is extended in both handshapes. The thumb makes contact with the nonselected fingers, which are bent in A, flattened in B.

Selected finger and thumb
The index, middle, or little finger is selected along with the thumb. The selected finger and the thumb vary in this type of handshapes in shapes and the presence of thumb-finger opposition. The selected finger and thumb, which are closed, extended, or rounded, differ in shape from the non-selected fingers.
3.1. No thumb-finger opposition; closed non-selected fingers 3.1a. Selected index finger and thumb 3.1b Selected index finger and thumb -complex handshapes In A, the thumb is in between the index and the middle finger. In B and C, the index finger is bent; the thumb is abducted and contacts the index finger at the level of the middle phalanx (B) or of the carpal-metacarpal joint (C).
In D, the index finger is bent, the thumb is extended, and the middle, ring and little fingers are extended and not spread. In E, the index finger is bent and the thumb is abducted, forming a kind of C shape; the non-selected fingers are rounded.
3.1c Selected middle finger and thumb The thumb is extended; the middle finger is flattened (A) or extended (B).

3.1d Selected little finger and thumb
The thumb and little finger are extended (A) or bent (B).
3.1e Selected little finger and thumb -complex handshape The selected little finger and thumb are extended; the non-selected fingers are flattened.

No thumb-finger opposition; extended non-selected fingers
The finger selected along with the thumb is either the index finger (A) or the middle finger (B and C). The selected finger is flattened (A and C) or bent (B). The thumb is abducted.

Two fingers selected
The selected fingers are the index and middle fingers (A), the index and little fingers (B), or the ring and little fingers (C).

Selected index and middle fingers
The index and middle fingers differ from the non-selected digits (thumb, ring and little fingers) that are closed. The two selected fingers vary depending on whether or not they are spread. They are shaped identically or differently, and their shapes vary (extended, bent, flattened, hooked).

4.1b Differently shaped index and middle fingers
In A, the selected index and middle fingers are flattened -in contrast to the non-selected fingers that are closedand the middle finger is above the index finger. In B, the selected middle and the ring fingers are flattened; the nonselected index and little fingers are extended.

Two fingers and thumb selected
The index and middle fingers or the index and little fingers are selected along with the thumb. The two selected fingers and the thumb are differently shaped compared to the non-selected fingers that are closed or stacked.

Selected and identically shaped index and middle fingers
The index and middle fingers do not make contact with the thumb, and are either spread or not spread. This type of handshapes vary for the configuration of the index and middle fingers (extended, bent, flattened, or hooked) as well as for thumb position (extended or abducted).